One by one, the jurors held the Glock 9-millimeter pistol that had belonged to Police Officer Peter Liang, from which a bullet ricocheted down a housing project stairwell and killed an unarmed man in November 2014.

In turn, each juror aimed the gun at the back wall of the courtroom and squeezed the trigger. Each click was audible. In the audience, some started in their seats.

The dramatic demonstration took place at State Supreme Court in Brooklyn on Tuesday, the second week of the trial of Officer Liang, who faces manslaughter and other charges in the death of Akai Gurley, 28. A bullet from the officer’s gun pierced Mr. Gurley’s heart in a stairwell at the Louis H. Pink Houses in the East New York section of Brooklyn.

Earlier on Tuesday, Melissa Butler, who was with Mr. Gurley on the staircase, described her attempts to revive him on the landing.

“I leaned over him in a puddle of blood and urine,” an emotional Ms. Butler said. “I was telling him to stay with me, I am getting him help. It was a soft voice.”

Officer Liang’s gun fired into the stairwell as he opened the door of the eighth floor staircase. Firearms experts who have previously testified described modifications made to police guns that increase the amount of pressure required to discharge a bullet. The jurors who elected to pull the trigger for themselves appeared to be testing just how much force needed to be applied for the gun to fire.

Ms. Butler, who described Mr. Gurley as her boyfriend, said she met Mr. Gurley at a shop near the Pink Houses in 2011, when he offered to pay for her purchases and she refused. They spent the evening of his death at her family’s apartment, on the seventh floor of 2724 Linden Boulevard, with her parents, her two sisters, her niece and her daughter. Ms. Butler played with Mr. Gurley and plaited his cornrows, she said. As Ms. Butler spoke, Kimberly Ballinger, with whom Mr. Gurley had lived in the Red Hook section of Brooklyn and with whom he was raising two children, looked away.

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Melissa Butler, who was with the victim, Akai Gurley, on the night of the shooting, left the courtroom after testifying.CreditBryan R. Smith for The New York Times

Marc Fliedner, an assistant prosecutor in the Brooklyn district attorney’s office, has focused on the fact that Officer Liang did not help Mr. Gurley and did not perform CPR, as is required of a police officer, arguing he was negligent in his duties. Only Ms. Butler tried to resuscitate Mr. Gurley, guided by a neighbor she had summoned, Melissa Lopez. Ms. Lopez relayed instructions from a 911 operator. Rae Downes Koshetz, a lawyer for Officer Liang, has said he was in shock and unable to render aid.

On Tuesday, the recording of the 911 call was played for the second time during the trial. As Ms. Butler heard her own voice and recounted pressing on her boyfriend’s chest and blowing in his mouth, her face began to crumble. A video was shown of Mr. Gurley’s bloodied clothes crumpled on the fifth floor landing.

“We’re almost done,” Joe Alexis, an assistant district attorney, said as Ms. Butler sobbed.

Late on Tuesday afternoon, Officer Shaun Landau, Officer Liang’s partner and a former classmate at the Police Academy, took the witness stand. Officer Landau previously testified before a grand jury and has been granted immunity. He recounted the night of the shooting at the Pink Houses complex — which was unremarkable until the pair decided to conduct a “vertical patrol,” a practice of inspecting public housing stairwells from top to bottom.

During the cross-examination, Robert E. Brown, another lawyer for Officer Liang, seemed to suggest that Officer Landau was testifying to protect himself.

“I am here to testify the facts of what I remember,” said Officer Landau, who has been on administrative duty since the shooting. “I am not here against my partner. I’m just a witness.”

In a police interview, part of which was read in court, Officer Landau said that Officer Liang, who also faces a criminal charge of official misconduct for not calling an ambulance or aiding Mr. Gurley, had in fact called for an ambulance just after the shooting. All calls are recorded over police radio. However, there is no recording of such a call, according to law enforcement sources familiar with the investigation.

In other testimony, Detective Joe Agosto, a firearms instructor with the New York Police Academy, said accidental gun discharges in the Police Department occurred about 20 times a year. Few, Detective Agosto said, were a result of malfunctions of the guns or triggers.

“The finger may gravitate toward it in certain circumstances,” he said under cross-examination by Mr. Brown.

“There is a belief that there is a tendency to reassure yourself that the trigger is there,” he added. “Comparable to touching your wallet.”

Correction:

An article on Wednesday about testimony at the manslaughter trial of Officer Peter Liang referred incorrectly in some editions to information provided by Officer Shaun Landau, Mr. Liang’s partner. Some parts of Officer Landau’s account, which was read in court, came from a police interview, not from grand jury testimony.

A version of this article appears in print on , on Page A21 of the New York edition with the headline: Gun Tests and Tears at Trial of Officer Accused in Killing. Order Reprints | Today’s Paper | Subscribe